foxhunt/crates/ml-alpha/tests/backward_finite_diff.rs

//! Finite-difference validation of cfc_step_backward + multi_horizon_heads_backward.
//!
//! Per `feedback_no_cpu_test_fallbacks.md`: the kernel is the reference.
//! We perturb a single input, re-run the GPU forward, and compare the
//! resulting finite-difference gradient to the analytic backward.

use approx::assert_relative_eq;
use ml_alpha::cfc::step::{cfc_step_backward_gpu, cfc_step_gpu, CfcWeights};
use ml_alpha::heads::{
    multi_horizon_heads_backward_gpu, multi_horizon_heads_gpu, HeadsWeights, HIDDEN_DIM, N_HORIZONS,
};
use ml_core::device::MlDevice;
use rand::{Rng, SeedableRng};
use rand_chacha::ChaCha8Rng;

fn test_device() -> MlDevice {
    MlDevice::cuda(0).expect("CUDA 0 required for ml-alpha tests")
}

fn rand_heads(seed: u64) -> HeadsWeights {
    let mut r = ChaCha8Rng::seed_from_u64(seed);
    HeadsWeights {
        w: (0..N_HORIZONS * HIDDEN_DIM).map(|_| r.gen_range(-0.1..0.1)).collect(),
        b: (0..N_HORIZONS).map(|_| r.gen_range(-0.01..0.01)).collect(),
    }
}

fn rand_cfc(seed: u64, n_in: usize, n_hid: usize) -> CfcWeights {
    let mut r = ChaCha8Rng::seed_from_u64(seed);
    CfcWeights {
        w_in: (0..n_hid * n_in).map(|_| r.gen_range(-0.1..0.1)).collect(),
        w_rec: (0..n_hid * n_hid).map(|_| r.gen_range(-0.05..0.05)).collect(),
        b: (0..n_hid).map(|_| r.gen_range(-0.01..0.01)).collect(),
        tau: (0..n_hid).map(|_| r.gen_range(0.05..2.0)).collect(),
        n_in,
        n_hid,
    }
}

#[test]
fn heads_backward_grad_h_matches_finite_diff() {
    let dev = test_device();
    let w = rand_heads(0x1111);
    let mut r = ChaCha8Rng::seed_from_u64(0x2222);
    let h: Vec<f32> = (0..HIDDEN_DIM).map(|_| r.gen_range(-1.0..1.0)).collect();
    let probs = multi_horizon_heads_gpu(&dev, &w, &h).unwrap();

    // Surrogate loss L = sum_k probs[k] -> dL/dp = 1 for all k.
    let grad_probs = [1.0f32; N_HORIZONS];
    let (_, _, grad_h_analytic) =
        multi_horizon_heads_backward_gpu(&dev, &w, &probs, &h, &grad_probs).unwrap();

    let eps = 1e-3_f32;
    for &i in &[0usize, 32, 64, 96, 127] {
        let mut h_up = h.clone();
        h_up[i] += eps;
        let mut h_dn = h.clone();
        h_dn[i] -= eps;
        let p_up = multi_horizon_heads_gpu(&dev, &w, &h_up).unwrap();
        let p_dn = multi_horizon_heads_gpu(&dev, &w, &h_dn).unwrap();
        let l_up: f32 = p_up.iter().sum();
        let l_dn: f32 = p_dn.iter().sum();
        let fd = (l_up - l_dn) / (2.0 * eps);
        assert_relative_eq!(grad_h_analytic[i], fd, epsilon = 1e-3, max_relative = 1e-2);
    }
}

#[test]
fn heads_backward_grad_b_matches_finite_diff() {
    let dev = test_device();
    let w = rand_heads(0x3333);
    let mut r = ChaCha8Rng::seed_from_u64(0x4444);
    let h: Vec<f32> = (0..HIDDEN_DIM).map(|_| r.gen_range(-1.0..1.0)).collect();
    let probs = multi_horizon_heads_gpu(&dev, &w, &h).unwrap();
    let grad_probs = [1.0f32; N_HORIZONS];
    let (_, grad_b_analytic, _) =
        multi_horizon_heads_backward_gpu(&dev, &w, &probs, &h, &grad_probs).unwrap();

    let eps = 1e-3_f32;
    for k in 0..N_HORIZONS {
        let mut w_up = w.clone();
        w_up.b[k] += eps;
        let mut w_dn = w.clone();
        w_dn.b[k] -= eps;
        let p_up = multi_horizon_heads_gpu(&dev, &w_up, &h).unwrap();
        let p_dn = multi_horizon_heads_gpu(&dev, &w_dn, &h).unwrap();
        let l_up: f32 = p_up.iter().sum();
        let l_dn: f32 = p_dn.iter().sum();
        let fd = (l_up - l_dn) / (2.0 * eps);
        assert_relative_eq!(grad_b_analytic[k], fd, epsilon = 1e-3, max_relative = 1e-2);
        let _ = w; // suppress
        let _ = w_up;
        let _ = w_dn;
    }
    let _ = w;
}

#[test]
fn cfc_backward_grad_b_matches_finite_diff() {
    let dev = test_device();
    let n_in = 8;
    let n_hid = 16;
    let w = rand_cfc(0x5555, n_in, n_hid);
    let mut r = ChaCha8Rng::seed_from_u64(0x6666);
    let x: Vec<f32> = (0..n_in).map(|_| r.gen_range(-1.0..1.0)).collect();
    let h_old: Vec<f32> = (0..n_hid).map(|_| r.gen_range(-1.0..1.0)).collect();
    let dt_s = 0.02_f32;

    // Surrogate loss L = sum_i h_new[i].
    let grad_h_new = vec![1.0f32; n_hid];
    let (_, _, grad_b_analytic, _) =
        cfc_step_backward_gpu(&dev, &w, &x, &h_old, &grad_h_new, dt_s).unwrap();

    let eps = 1e-3_f32;
    for &i in &[0usize, 4, 8, 15] {
        let mut w_up = w.clone();
        w_up.b[i] += eps;
        let mut w_dn = w.clone();
        w_dn.b[i] -= eps;
        let h_up = cfc_step_gpu(&dev, &w_up, &x, &h_old, dt_s).unwrap();
        let h_dn = cfc_step_gpu(&dev, &w_dn, &x, &h_old, dt_s).unwrap();
        let l_up: f32 = h_up.iter().sum();
        let l_dn: f32 = h_dn.iter().sum();
        let fd = (l_up - l_dn) / (2.0 * eps);
        assert_relative_eq!(grad_b_analytic[i], fd, epsilon = 5e-3, max_relative = 5e-2);
    }
}

#[test]
fn cfc_backward_grad_h_old_matches_finite_diff() {
    let dev = test_device();
    let n_in = 8;
    let n_hid = 16;
    let w = rand_cfc(0x7777, n_in, n_hid);
    let mut r = ChaCha8Rng::seed_from_u64(0x8888);
    let x: Vec<f32> = (0..n_in).map(|_| r.gen_range(-1.0..1.0)).collect();
    let h_old: Vec<f32> = (0..n_hid).map(|_| r.gen_range(-1.0..1.0)).collect();
    let dt_s = 0.02_f32;
    let grad_h_new = vec![1.0f32; n_hid];
    let (_, _, _, grad_h_old_analytic) =
        cfc_step_backward_gpu(&dev, &w, &x, &h_old, &grad_h_new, dt_s).unwrap();

    let eps = 1e-3_f32;
    for &i in &[0usize, 4, 8, 15] {
        let mut h_up = h_old.clone();
        h_up[i] += eps;
        let mut h_dn = h_old.clone();
        h_dn[i] -= eps;
        let h_new_up = cfc_step_gpu(&dev, &w, &x, &h_up, dt_s).unwrap();
        let h_new_dn = cfc_step_gpu(&dev, &w, &x, &h_dn, dt_s).unwrap();
        let l_up: f32 = h_new_up.iter().sum();
        let l_dn: f32 = h_new_dn.iter().sum();
        let fd = (l_up - l_dn) / (2.0 * eps);
        assert_relative_eq!(grad_h_old_analytic[i], fd, epsilon = 5e-3, max_relative = 5e-2);
    }
}